1. Field of the Invention
The present invention relates generally to the use of radio frequency energy for heating and necrosing solid tissue. More particularly, the present invention relates to a control method and system for delivering radio frequency current to the tissue through an electrode or electrode array disposed within the tissue.
The delivery of radio frequency energy to target regions within solid tissue is known for a variety of purposes. Of particular interest to the present invention, radio frequency energy may be delivered to diseased regions in target tissue for the purpose of tissue heating and/or necrosis, referred to generally as hyperthermia. For example, the liver is a common depository for metastases of many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney and lung. Electrosurgical probes for deploying single and multiple electrodes have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. See, for example, the references cited in the Description of the Background Art hereinafter.
A primary goal in such hyperthermic treatments, particularly those intended for tumor treatment, is the complete, thorough, and uniform heating of the target tissue mass. Uniform heating of the tissue mass, however, can be difficult to achieve, particularly in highly vascularized tissues where variability in local blood flow can have a significant effect on the heating characteristics of the tissue. For example, creation of a lesion having a selected volume in some highly perfused tissue locations may require twice as much power as an identically-sized lesion in less highly perfused locations. While a variety of approaches for achieving such complete, thorough, and uniform heating of tissue have been proposed, most such approaches are somewhat complex and require the use of electrodes which are capable of measuring temperature, impedance, or the like. See, for example, the radio frequency power supply described in published PCT Application WO 93/08757. In general, many approaches for achieving uniform tissue heating have relied on slow, gradual heating of the tissue in order to avoid the formation of charred or otherwise necrosed, high radio frequency impedance regions within the target tissue mass. Such approaches, however, are complex, can result in an undesirable prolongation of the treatment, and are not always successful,
For these reasons, it would be desirable to provide improved treatment methods, systems, and apparatus which allow for effective and efficient delivery of a radio frequency energy to solid tissue masses using electrodes. In particular, it would be desirable to provide such methods, systems, and apparatus which are useful with many or all tissue-penetrating electrode systems which are now available or which might become available in the future. The methods, systems, and apparatus should be simple to implement and use, and should preferably reduce the complexity, cost, and treatment time required to achieve complete heating and/or necrosis of the target tissue mass. The methods, systems, and apparatus should preferably require no information or feedback from the tissue region being treated, other than information which can be acquired from the power delivery characteristics which can be monitored in the radio frequency power delivery system itself. In particular, the methods, systems, and apparatus should be able to operate solely by monitoring the power and/or current delivery characteristics of the radio frequency energy into an electrode system present in the target tissue. At least some of these objective will be met by the present invention as claimed hereinafter.
2. Description of the Background Art
The heating of solid tissue with radio frequency current using the preferred electrode structures of the present invention is described in WO 96/29946 and co-pending Applications Ser. Nos. 08/410,344; 08/559,072; 08/766,154; 08/764,085; and 08/858,414, filed on May 19, 1997, the full disclosures of which are incorporated herein by reference.
WO 97/06739; WO 97/06740; WO 97/06855; and WO 97/06857 describe RF treatment electrodes and note that power delivery can xe2x80x9cimpede outxe2x80x9d if levels are raised too high.
Assignee of the present application has developed a radio frequency power supply (Model RF-2000, Radio Therapeutics Corporation, Mountain View, Calif.) which provides power levels up to 100 W and is intended for the coagulation (ablation) of soft tissue. The power supply is controlled by a programmable microprocessor which is capable of continuously monitoring power delivered to an electrode system.
Patents and published applications describing radio frequency tissue ablation using electrodes having various configurations include U.S. Pat. Nos. 5,662,680; 5,599,346; 5,599,345; 5,562,703; 5,536,267; 5,489,161; 5,472,441; and 5,458,597; and published International Applications WO 97/06857; WO 97/06855; WO 97/06740; WO 97/06739; WO 96/04860; and WO 95/13113.
A radio frequency power supply having impedance monitoring capability is described in WO 93/08757.
Other radio frequency power apparatus and methods are described in U.S. Pat. Nos. 5,556,396; 5,514,129; 5,496,312; 5,437,664; and 5,370,645; and WO 95/20360, WO 95/09577, and WO 95/20360.
The present invention provides improved methods, systems, and apparatus for delivering radio frequency energy to electrodes disposed in tissue for inducing hyperthermia and other purposes. It has been found that the delivery of radio frequency power to electrode(s) disposed in tissue can, if the power is delivered for a sufficient time and/or at a sufficient power delivery level or flux, result in an abrupt increase in the electrical impedance between the electrode(s) and tissue. While such an abrupt increase in impedance is undesirable since it results in an immediate fall-off of energy delivery (for a voltage limited radio frequency power source), the present invention relies on the occurrence of the abrupt reduction in power delivery (which may be observed as a reduction in current delivery to the electrodes) to provide information about the heat capacity and heat delivery characteristics of the local target tissue region. The present invention uses such information to control subsequent delivery of energy to the target tissue region using the same electrode(s).
The present invention still further depends, in least in part, on the observation that the abrupt rise in the electrode-tissue interface impedance diminishes very rapidly when the power delivery is stopped, typically disappearing within several seconds. Delivery of the radio frequency power can be resumed after the impedance has diminished, typically to impedance levels substantially equal to those observed prior to the abrupt increase. Based on these observations, it is possible to determine improved or optimized radio frequency power delivery levels and protocols based on the power levels and/or time periods required to induce the abrupt impedance increases and associated power declines in specific target locations. In particular, the protocols rely on appropriate adjustments to the power levels which are resumed after the tissue impedance diminishes.
It is presently believed that the abrupt increase in electrode-tissue interface impedance results from the formation of a thin gaseous layer over the electrode surface, apparently resulting from vaporization of water within the tissue as the temperature approaches the local boiling point. Surprisingly, the thin gaseous layer appears to spread from an initial nucleation site to cover most or all of an electrode surface in a very short time period, typically less than 30 seconds, resulting in an increase in electrode-tissue interface impedance which is very large when compared to the total system impedance prior to formation of the thin gaseous layer. In the exemplary systems described herein after, typical system impedance prior to formation of the thin gaseous layer will be in the range from 40 Ù to 70 Ù, which impedance will rise to from 300 Ù to 400 Ù after formation of the thin gaseous layer. While this is presently believed to be the mechanism responsible for the above-described observations, the present invention does not depend on the accuracy of this model. The methods, systems, and apparatus of the present invention have been found to be useful and effective regardless of the actual mechanism which is responsible for the change in impedance.
In a first particular aspect of the present invention, a method for heating tissue and/or controlling the delivery of radio frequency energy to an electrode and tissue comprises gradually increasing the power delivery rate to the tissue over time until an abrupt decrease in the power delivery rate (resulting from the increase in electrode-tissue interface impedance) is observed. The power which such power drop occurs can be determined, and is considered a xe2x80x9cmaximumxe2x80x9d power level which should not be exceeded. After waiting for the electrode-tissue impedance to return to an acceptable level, typically requiring fifteen seconds or less, the electrodes can be reenergized and the power delivery to tissue resumed at a level which is some fraction of the maximum power delivery rate. Typically, the reenergization power level is from 50% to 90% of the maximum power level, preferably being from 70% to 80%. The increase in the power delivery rate used to determine the maximum power level will depend on the electrode size as well as the initial power level, typically being linear and increasing at a rate in the range from 1 W/min. to 25 W/min., preferably from 5 W/min. to 10 W/min.
In a second aspect of the present invention, a method for determining the subsequent level of power delivery relies on rapidly increasing the level of power delivered to the tissue to an amount which has been predetermined to certainly or very likely produce the abrupt impedance increase/power delivery decrease almost immediately after the power increase is initiated, typically within 10 seconds, preferably within 5 seconds. While the time between initiating the power increase and observing the power decrease will be relatively short, it will be finite and measurable. The elapsed time between such initiation and the observed power decrease is relied on by the present invention as an indicator of the margin between an equilibrium or gradually increasing electrode/tissue interface temperature and a maximum electrode/tissue interface temperature characteristic of that particular target tissue location.
By establishing an initial level of radio frequency power delivered to the target tissue mass, a relatively stable (equilibrium) or gradually increasing electrode/tissue interface temperature is achieved. By then rapidly increasing or pulsing the energy delivery rate to induce the impedance increase/power decrease, the time required to induce the event provides a qualitative determination of how close the initial treatment temperature was to a xe2x80x9cmaximumxe2x80x9d characteristic of that tissue location. This qualitative determination, in turn, can be relied on for raising, lowering, or maintaining the initial level of power delivery to the tissue. In particular, if the power decrease/impedance increase occurs almost immediately after the delivered power is rapidly increased, e.g. within 2 seconds to 5 seconds, usually from 2 seconds to 3 seconds, it can be assumed that the treatment conditions prior to the power increase had resulted in a tissue temperature which is very close to the maximum. Thus, in order to achieve uniform and complete heating of the tissue, it will be desirable to decrease the power delivery rate to the tissue from the initial rate to delay formation of the thin gaseous layer. Conversely, if the power decrease/impedance increase requires a relatively long time period to occur, for example from 10 seconds to 20 seconds, it can be assumed that the initial tissue temperature is relatively far from the maximum sustainable by the tissue. Thus, it will be desirable to increase the power delivery rate in order to achieve optimum tissue hyperthermia. There may also be instances, of course, where the elapsed time between power increase to power decrease/tissue impedance increase will be considered within an acceptable or optimum range, where the initial treatment power level need not be modified. In this second aspect of the present invention, it can be seen that the power level used for the subsequent treatment of the tissue mass will vary inversely with respect to the observed length of the elapsed time.
In general, the radio frequency energy will be supplied as a radio frequency current using a controlled voltage or constant voltage power supply. The use of such radio frequency power sources is preferred because the limited voltage available necessarily results in a reduction of current when the electrode/tissue impedance rises. In addition to allowing the monitoring of impedance based on observing the power or current delivered to the electrodes, the limited voltage also decreases the likelihood of arcing or sparking from the electrode into the tissue. Usually, the power supply will be operated at a level which depends on the size of the electrode, the target tissue type, and the degree of tissue perfusion. Typically, the power supply will provide power in the range from 10 W to 200 W, during all phases of the above-described methods. For prolonged treatment, the electrodes will generally be energized at a power between 20 and 100 W which is in the range from 50% to 90% of the local maximum power level, usually from 70% to 80%.
In further aspects of the present invention, systems are provided which comprise an electrosurgical power supply, typically a radio frequency power supply, in combination with written, electronic, or other instructions setting forth any of the methods set forth above.
In still another aspect of the present invention, computer programs embodied in a tangible medium, such as a floppy disk, compact disk, tape, flash memory, hard disk memory, or the like, which set forth any of the methods described above, in computer-readable code. Such computer programs are useful with digital controllers which may be built into a radio frequency power supply or other electrosurgical power supply according to the present invention. Alternatively, such programs may be useful with general purpose computers, such as personal computers, which can be interfaced with conventional electrosurgical power supplies for the control thereof according to any of the methods of the present invention.
In a still further aspect of the present invention, electrosurgical power supplies are provided which comprise a radio frequency power source having a voltage controlled output, a connection for a tissue electrode, and a connection for a return or counter electrode. The electrosurgical power supplies will further comprise a digital controller or other means for automatically adjusting the power output of the power supply, where the power is delivered between an electrode and a counter electrode, where the electrodes are coupled to the power supply and present in solid tissue. The controller or other adjusting means is programmed or programmable to automatically increase power delivered by the radio frequency power source into a target tissue mass to a maximum level where an abrupt rise in impedance occurs. After the abrupt rise is observed, the controller or other adjusting means will reduce the power delivered by the radio frequency power source to a level below that at which the increase in impedance is maintained. After allowing the impedance to decrease, a controller or other adjusting means will increase or reestablish a treatment power level which is below the maximum level observed, but which may be higher or lower than any initial treatment level employed before the power is pulsed. In particular, the controller or other adjusting means can be programmed to implement any of the methods described above independent of operator intervention.